1) Field of the Invention
The present invention relates to a process for producing diaryl carbonate, and specifically, to a process for producing diaryl carbonate using urea as raw material via dialkyl carbonate as intermediate product. Diaryl carbonate is a compound useful as a raw material for the production of polycarbonate according to a molten transesterification method.
2) Prior Art
Hitherto, diaryl carbonate has been produced by the reaction of an aromatic hydroxy compound with phosgene. However, a process for producing diaryl carbonate without using phosgene has been required since phosgene has high toxicity and high corrosion behavior for apparatus and furthermore a large amount of alkali is necessary to neutralize by-produced hydrogen chloride. Therefore, some processes for producing diaryl carbonate without using phosgene have been attempted.
For example, a process which comprises conducting carbonylation of an aromatic hydroxy compound with oxidation by using both carbon monoxide and oxygen has been suggested (Japanese Patent Kokai (Laid-open) No.53-68744). However, the process includes a very complicated reaction system since very expensive palladium is used as main catalyst and furthermore cocatalyst, drying agent, oxidizing agent, etc., also are required. Further, under the present situation, it is difficult to recover catalyst and the process does not reach to an industrial level also in respect of yield and reaction rate.
On the other hand, it has been attempted to produce diphenyl carbonate by direct reaction of urea with phenol, but its yield is low (Japanese Patent Kokai (Laid-open) No. 8-92167). Further, it is known to convert urea to diphenyl urea and further convert it into phenyl urethane and then allow to conduct disproportionation of phenyl urethane, but the operation is intricate (Japanese Patent Kokai (Laid-open) No. 8-198815).
As processes for producing diaryl carbonate other than the above-mentioned processes, Japanese Patent Kokai (Laid-open) No. 51-105032 discloses a process for producing diphenyl carbonate by the reaction of phenol with dimethyl carbonate under an atmospheric pressure or an applied pressure in the presence of a catalyst selected from the group consisting of Lewis acid and transition metal compounds capable of forming Lewis acid, but the selectivity to dephenyl carbonate is low.
Japanese Patent Kokai (laid-open) No. 51-75044 discloses a process for producing diphenyl carbonate by contacting phenyl methyl carbonate or phenyl ethyl carbonate with a catalyst composition selected from the group consisting of Lewis acid and transition metal compounds capable of forming Lewis acid, but the conversion rate of pheny methyl carbonate or pheny ethyl carbonate is low.
Further, in the production of diphenyl carbonate from dimethyl carbonate, separation between dimethyl carbonate and methanol has been become very problematic since dimethyl carbonate forms an azeotrope with methanol. In order to avoid such azeotropy, every endeavor has been made (Japanese Patent Kokai (Laid open) Nos. 54-48732, 61-291545 and 7-330687, etc.). However, the fundamental problem that reaction efficiency is inferior, has not yet been solved since dialkyl carbonate as the raw material is slipped out during the reaction.
As processes for synthesis of dialkyl carbonate, (1) an oxidative carbonylation process which comprises reacting alcohol with both carbon monoxide and oxygen (Japanese Patent Kokai (Laid-open) No. 51-138620), (2) a nitrite ester process which comprises reacting a nitrite ester with carbon monoxide (Japanese Patent Kokai (Laid-open) No. 3-141243), (3) an ethylene carbonate process which comprises allowing to conduct transesterification between ethylene carbonate and alkyl alcohol (Japanese Patent Kokai (Laid-open) No. 8-176071 and (4) a urea process which comprises reacting directly urea with alkyl alcohol (Japanese Patent Kokai (Laid-open) Nos. 55-102542 and 8-119907).
In above-mentioned (1) the oxidative carbonylation process and (2) the nitrite ester process, the reaction apparatus becomes complicated since toxic CO is used. (3) The ethylene carbonate process includes both a process using ethylene oxide and carbon dioxide and a process using urea and ethylene glycol, as the raw material of ethylene carbonate. The process using ethylene oxide as the raw material is not preferable since ethylene glycols are produced together. The process using urea as the raw material is more complicated than (4) the process for producing directly dialkyl carbonate from urea.
In a process described in Japanese Patent Kokai (Laid-open) No. 55-102542 corresponding to (4) the process which comprises reacting directly urea with alkyl alcohol, dialkyl carbonate is obtained by using a higher alcohol having 8 or above carbon atoms. Japanese Patent Kokai (Laid-open) No. 8-119907 discloses a process for producing dialkyl carbonate from methanol or ethanol, but its yield is very low.
Thus, under the present situation, no process in which diaryl carbonate can be produced in a high yield by simple operation without using toxic phosgene, has yet been established.